Scanning probe microscopes are a class of imaging techniques in which a tip that interacts locally with a sample is scanned over the surface of the sample to generate a three-dimensional image representing the properties of the surface. For example, in atomic force microscopy, the surface interaction force between the probe tip and the sample are measured at each point on the sample. The tip has a very small end and is mounted on the end of a cantilevered arm. As the tip is moved over the surface of the sample, the arm deflects in response to the changes in topology of the surface. Images are typically acquired in one of two modes. In the contact or constant force mode, the tip is brought into contact with the sample and the tip moves up and down as the tip is moved over the surface. The deflection of the arm is a direct measure of force and topographical variations. A feedback controller measures the deflection and adjusts the height of the probe tip so as to maintain constant force between the cantilevered probe and the surface, i.e., the arm at a fixed deflection. The height of the probe tip as a function of position can then be used to create an “image” of the surface of the sample.
In the AC, or non-contact mode, the tip and arm are oscillated at a frequency near the resonant frequency of the arm. The height of the tip can be controlled such that the tip avoids contact with the sample surface, sampling short-range tip/sample forces. Alterations in the oscillation frequency from short range forces between the tip and the sample result in changes in the oscillations of the tip. Alternatively, the tip can be allowed to make light intermittent contact with the sample only at the bottom of the oscillation cycle. Contact between the probe tip and the sample results in an alteration of the amplitude, phase and/or frequency of the oscillation. The controller adjusts the height of the probe over the sample such that the oscillation amplitude, phase and/or frequency is kept at a predetermined constant value. Since the tip is not in constant contact with the sample, the sheer forces applied to the sample are significantly less than in the mode in which the tip is in constant contact. For soft samples, this mode reduces the damage that the tip can inflict on the sample and also provides a more accurate image of the surface in its non-disturbed configuration.
In all of these modes, the image is constructed one point at a time and limited by the rate at which the tip can be moved relative to the sample, as well as the time required for the servo loop to reposition the tip vertically to maintain the distance between the surface and the tip. Hence, the time to acquire an entire image can be several minutes or longer, since the image acquisition process depends on mechanically moving the sample being scanned relative to the measurement probe. In one class of system, the probe is moved over the sample in a raster scanning pattern that zig-zags back and forth over the sample until the entire sample area has been measured. The acquisition time depends on the resolution desired in the image; at high resolutions, the total scanning time can be very long. Such long acquisition times are tolerable for stationary samples that do not change over the long sample acquisition time. However, the use of scanning probe microscopy on dynamic systems, as in the case of measuring transient events in biological samples is inhibited by excessive sampling time, since the phenomena of interest often occur in times that are small compared to the image acquisition time.
Hence, scanning schemes that reduce the total scanning time have been sought. For example, more complex servo control algorithms are utilized to minimize the response time for tracking changes in topography at each measurement point on the sample. In a raster scan mode, the tip is moved rapidly in one direction and more slowly in the other direction. To simplify the following discussion, it will be assumed that the fast direction is the x-direction and that the slow direction is the y-direction. The repetitive motion generates signals at the x scan frequency and harmonics thereof in the detection signal. These harmonics can interfere with the tip height servo mechanism, and hence, lead to errors in the height measurements or a requirement that the scan speed be significantly reduced to reduce these errors. It is understood that there is repetitive motion along the slower y-direction and that this can also result in repetitive errors in the detected signal that can be addressed using this invention.